feat(engine): 实现阵营技能树系统

- 新增 FactionSkill、FactionSkillTree、FactionSkills 数据模型
- 创建 faction_skills.py 模块处理技能解锁和效果应用
- 技能解锁条件：power >= cost 且前置技能已解锁
- 技能效果：increase_positive_emotion、reduce_conflict、increase_fear、suppress_others
- 添加 4 个技能树测试用例，全部 30 个测试通过

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

engine-python/tests/test_step.py

@@ -481,3 +481,63 @@ def test_votes_trigger_faction_event(client):
 
     # 要么触发了事件，要么能量在累积
     assert triggered or factions["optimists"]["power"] > 0
+
+
+# ==================== 技能树系统测试 ====================
+
+def test_faction_skills_exists_in_world_state(client):
+    """测试 world_state 包含 faction_skills 字段"""
+    resp = client.post("/step", json={"events": []})
+    data = resp.json()
+
+    assert "faction_skills" in data["world_state"]
+    skills = data["world_state"]["faction_skills"]
+    assert "optimists" in skills
+    assert "fearful" in skills
+
+
+def test_faction_skills_has_required_fields(client):
+    """测试技能包含必要字段"""
+    resp = client.post("/step", json={"events": []})
+    data = resp.json()
+
+    skills = data["world_state"]["faction_skills"]
+    opt_skills = skills["optimists"]["skills"]
+
+    assert "festival_boost" in opt_skills
+    skill = opt_skills["festival_boost"]
+    assert "unlocked" in skill
+    assert "cost" in skill
+    assert "effect" in skill
+    assert "requires" in skill
+
+
+def test_skill_unlock_when_power_sufficient(client):
+    """测试能量足够时解锁技能"""
+    # 多轮投票累积能量到 10+
+    for i in range(6):
+        events = [
+            {"type": "vote", "faction": "optimists", "text": "", "user": f"u{i}_a", "ts": i},
+            {"type": "vote", "faction": "optimists", "text": "", "user": f"u{i}_b", "ts": i+0.1},
+        ]
+        client.post("/step", json={"events": events})
+
+    # 再执行一轮让投票生效
+    resp = client.post("/step", json={"events": []})
+    data = resp.json()
+
+    skills = data["world_state"]["faction_skills"]["optimists"]["skills"]
+    # festival_boost cost=10，应该已解锁
+    assert skills["festival_boost"]["unlocked"] is True
+
+
+def test_skill_requires_prerequisite(client):
+    """测试技能需要前置技能"""
+    resp = client.post("/step", json={"events": []})
+    data = resp.json()
+
+    skills = data["world_state"]["faction_skills"]["optimists"]["skills"]
+    # unity 需要 festival_boost 作为前置
+    assert skills["unity"]["requires"] == ["festival_boost"]
+    # 初始状态下 unity 未解锁
+    assert skills["unity"]["unlocked"] is False